On Tue, 01 Jun 2010 21:11:51 -0700, Kevin Normoyle wrote:
>You also showed that 12-16 turns of #14 THHN on a single type 31 core
>can have a much flatter response than the coax chokes, right?
They have a higher choking Z over a wider frequency range.
>You never showed that 10-12 turns of RG-400 wrapped normally with say 1"
>coils on the same type 31 core, is worse than the turn count you gave
>for #14 THHN. on the same core. At least not that I can tell?
No, I've not taken a serious look at any specialty coaxes that can be
wound very tightly.
>If you did, then you've shown that bifilar is better, with a few more
>turns. (for 3.5mhz-30mhz).
>Even though we can agree the measurement technique by balundesigns might
>be suspect, you have no data that their 10-12 turn small coax design is
>bad, unless you're confident they're not using type 31 ferrite.
I've looked at a lot of websites that folks have pointed me to. I don't
recall which theirs specifically. I don't need to buy any chokes. :) I
don't recall seeing any, based on what I could see, that I thought were
using lossy ferrites as cores. In other words, they have the same problem
as other chokes wound on #61. I do know that Array Solutions is using #31
for some products he sells. A year ago, Jay called me to thank me for
pointing him to that material.
>I want something with more voltage breakdown than the PVC on THHN. So I
>have my two choices: bifilar #16 teflon awg and RG400, wound some number
>of times around some number of cores.
How much power are you running? In a 1:1 choke, a choke wound with THHN
will handle 1200 volts. 1.5kW at 50 ohms is 275 volts. At 200 ohms its 548
volts, and that's divided between the two chokes. That's a LOT of headroom
for voltage!
>You've not run your #14 THHN at 1500 watts for any period of time.
>Anecdotal info seems to suggest eventually I might have a problem. Even
>though it's fine for matched swr, what happens when you accidently drive
>1500 watts into the wrong antenna?
I've run it PLENTY long enough to know that there's very little heating in
a choke that is exposed to air.
>I read your paper, and I think you're showing bifilar is superior to the
>coax chokes you have, but it's unclear whether small diameter coax is as
>bad as you suggest, with the same winding/core. If it is, then it's
>obvious I should go bifilar.
I'm showing that bifilar windings are a very effective low cost
alternative to winding big coax through multiple cores or the biggest
clamp-ons. A major reason for doing so is that one of my friends in the UK
says that they are experiencing exhorbitantly high mark-ups on Fair-Rite
cores.
>Am I misunderstanding?
Yes. One size does not fit all. Each choke has advantages and
disadvantages. That's why the choke cookbook makes different
recommendations for each band or combinations of bands that your antenna
covers. THHN puts a bump in the impedance of a 50 ohm system. Coax and a
enameled bifilar pair do not. There are some who are bothered by that,
some who are not.
A single specification does not tell us all that we need to know. There's
the issue of impedance. Winding coax through a stack of cores can
withstand weather without any protection. I would want some sort of
enclosure for a bifilar winding on a toroid, you need connectors on it,
etc. So it's a minor construction project. A choke inside an enclosure
will get warmer, but I don't think it would get hot enough to be a problem
at legal ham power levels. The core doesn't get hot -- the heating is in
the wire.
73,
Jim K9YC
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